Multimode Rayleigh wave inversion for heterogeneity and
azimuthal anisotropy of the Australian upper mantle
1 Department of Earth, Atmospheric and Planetary
Massachusetts Institute of Technology (MIT)
Cambridge MA 02139,
2 Laboratoire de Sismologie
Geophysical Journal International, 2002, 151 (3), 738-754,
Physique du Globe Paris (IPGP)
We present an azimuthally anisotropic three-dimensional
shear-wave speed model for the Australian upper mantle obtained
from the dispersion of fundamental and higher modes of Rayleigh
waves. We compare two tomographic techniques to map path-average
Earth models into a 3-D model for heterogeneity and azimuthal
anisotropy. Method I uses a rectangular surface cell
parameterization and depth basis functions that represent
independently constrained estimates of radial Earth
structure. It performs an iterative inversion norm damping and
gradient regularization. Method II uses a direct inversion of
individual depth layers constrained by Bayesian assumptions
about the model covariance. We recall that Bayesian inversions
and discrete regularization approaches are theoretically
equivalent, and with a synthetic example we show that they can
give similar results. The model we present here uses the
discrete regularized inversion of independent path constraints
of Method I, on an equal-area grid. With the exception of
westernmost Australia, we can retrieve structure on length
scales of about 250 km laterally and 50 km in the radial
direction, to within 0.8% for the velocity, 20% for the
anisotropic magnitude and 20 degrees for its direction. On
length scales of 1000 km and longer, down to about 200 km, there
is a good correlation between velocity heterogeneity and
geologic age. At shorter length scales and at depths below 200
km, however, this relationship breaks down. The observed
magnitude and direction of maximum anisotropy do not, in
general, appear to be correlated to surface geology. The pattern
of anisotropy appears to be rather complex in the upper 150 km,
whereas a smoother pattern of fast axes is obtained at larger
depth. If some of the deeper directions of anisotropy are
aligned with the approximately N--S direction of absolute plate
motion, this correspondence is not everywhere obvious, despite
the fast (7 cm/yr) northward motion of the Australian
plate. This suggests that other mechanisms are important in
creating the deeper anisotropy. Predictions of SKS splitting
times and directions, an integrated measure of anisotropy, are
poorly matched by observations of shear-wave birefringence.
- Figure 01 Distribution of stations and events used in this study.
- Figure 02 Equal-area, irregular, and tectonic grids used in the inversion.
- Figure 03 Synthetic test with uniform path coverage: input.
- Figure 04 Synthetic test with uniform path coverage: recovery with Bayesian and regularization techniques.
- Figure 05 Exact resolution computation for synthetic and real data sets.
- Figure 06 Checkerboard test with azimuthal anisotropy..
- Figure 07 Checkerboard input/output test at different depths.
- Figure 08 Trade-off of heterogeneity and anisotropy at 140 km.
- Figure 09 Inversion results at different depths.
- Figure 10 Inversion results at different depths: cross-sections
- Figure 11 Regionalized presentation of heterogeneity and anisotropy
- Figure 12 Predictions and observations of shear-wave splitting.
Last modified: Wed Apr 12 23:06:25 EDT 2023